Numerical Analysis of the Primary Gas Boundary Layer Flow Structure in Laser Fusion Cutting in Context to the Striation Characteristics of Cut Edges

In cutting metals with solid-state lasers, a characteristic cutting edge structure is generated whose formation mechanisms still elude a consistent explanation. Several studies suggest a major contribution of the pressurized gas flow. Particular emphasis must be devoted to the gas boundary layer and its developing flow characteristics, since they determine the heat and momentum exchange between the cutting gas and the highly heated melt surface and thus the expulsion of the molten material from the kerf. The present study applies a CFD simulation model to analyze the gas flow during laser cutting with appropriate boundary conditions. Specifically, the gas boundary layer development is considered with a high spatial discretization of this zone in combination with a transition turbulence model. The results of the calculation reveal for the first time that the boundary layer is characterized by a quasi-stationary vortex structure composed of nearly horizontal geometry- and shock-induced separation zones and vertical vortices, which contribute to the transition to turbulent flow. Comparison of the results with the striation structure of experimental cut edges reveals a high agreement of the location, orientation, and size of the characteristic vortices with particular features of the striation structure of cut edges.

Analysis of process efficiency in laser fusion cutting and some single- and multi-objective optimization aspects

For an efficient use of laser cutting technology, it is of great importance to analyze the impact of process parameters on different performance indicators, such as cut quality criteria, productivity criteria, costs as well as environmental performance criteria (energy and resource efficiency). Having this in mind, this study presents the experimental results of CO2 laser fusion cutting of AISI 304 stainless steel using nitrogen, with the aim of developing a semi-empirical mathematical model for the estimation of process efficiency as an important indicator of the achievable energy transfer efficiency in the cutting process. The model was developed by relating the theoretical power needed to melt the volume per unit time and used laser power, where the change of kerf width was modeled using an empirical power model in terms of laser cutting parameters such as laser power, cutting speed, and focus position. The obtained results indicated the dominant effect of the focus position on the change in process efficiency, followed by the cutting speed and laser power. In addition, in order to maximize process efficiency and simultaneously ensure high cut quality without dross formation, a laser cutting optimization problem with constraints was formulated and solved. Also, a multi-objective optimization problem aimed at simultaneous optimization of process efficiency and material removal rate was formulated and solved, where the determined set of Pareto non-dominated solutions was analyzed by using the entropy method and multi-criteria decision analysis method, that is, the Technique for Order of Preference by Similarity to Ideal Solution. The optimization results revealed that in order to enhance process efficiency and material removal rate, while ensuring high cut quality without dross formation, focusing the laser beam deep into the bulk of material is needed with particular trade-offs between laser power and cutting speed levels at high pressure levels of nitrogen.

Three-dimensional synchronous proton radiography for dynamic magnetic fields in laser-produced high-energy-density plasmas

Understanding the generation and evolution of magnetic fields in high-energy-density plasmas is a major scientific challenge in broad research areas including astrophysics, cosmology, and laser fusion energy. However, the fully three-dimensional (3D) topologies of such dynamic magnetic fields are still unknown yet. Here we report experiments of the first 3D synchronous proton radiography for self-generated magnetic fields in respectively laser-produced low-Z CH and high-Z Cu plasmas. The radiography images show that abundant 3D filamentary structures of magnetic fields grow up in coronal region of CH plasmas, while for Cu, the fields are majorly compressed along the dense surface region whose internal structures are pretty vague. These results are reproduced and explained by a combination of radiation-magnetohydrodynamic, particle-in-cell and Vlasov-Fokker-Planck simulations, where the cross-scale effects of Biermann battery, Nernst advection, resistive diffusion, Righi-Leduc and particularly kinetic Weibel instability are all taken into account. Our findings provide much enlightenment to the role of magnetic field generation in implosion and hohlraum dynamics of laser fusion.

Research on Self-Aligning Flanges Based on Piezoelectric Actuators Applied to Precision Grinding Machines

Laser fusion research requires a large number of high-precision large-diameter aspherical components. To improve the grinding efficiency in the component production process, the manual operation time during the grinding process needs to be reduced. The grinding process requires the installation of the dressed grinding wheel onto the grinding machine spindle, and the off-line dressing results in installation errors during the loading and unloading process, which requires more time for manual alignment. To achieve self-aligning, the circumferential contour of the grinding wheel was first restored by the reversal method, then noise reduction and circle fitting were performed to obtain the eccentricity value and eccentricity position between the flange and the spindle, and finally, the flange was adjusted finely by three piezoelectric actuators installed inside the flange to reduce the eccentricity. Three repetitive experiments were conducted to verify that the self-aligning flange can reduce the eccentricity value by retracting the piezoelectric actuators so that the proper alignment between the flange and the spindle could meet the requirements; the average eccentricity value of the three experiments decreased by 74%, which greatly improved the efficiency of the grinding wheel alignment.

Tephrostratigraphy, Magnetostratigraphy and Geochronology of Some Early and Middle Pleistocene Deposits in New Zealand

<p>Numerous early Pleistocene silicic tephras are exposed in long sedimentary sequences in the East Coast and Wanganui basin regions in southern North Island of New Zealand, some 150-250 km south of the Taupo Volcanic Zone. They provide time planes that can be correlated between different facies and basins. Individual tephras can often be distinguished on the basis of major and trace element glass chemistry, and Fe-Ti oxide composition. Approximately 51 different eruptive events may be recorded in the interval from ca. 1.7 Ma to 0.5 Ma. Early Pleistocene tephras in deep-sea sediments of the Southern Pacific Ocean at latitudes >60 degrees S were previously considered to have been sourced in the TVZ. However, their alkalic compositions are compatible only with volcanoes of Western Antarctica and the Ross Sea region. Most of the tephras examined here are reworked, and many have been emplaced as catastrophic flood deposits in overbank settings of braid plains in the East Coast region. Their mode of emplacement and the presence of ignimbrites in the sequences indicate early Pleistocene transport routes through the site of the present main Axial Ranges, and suggest substantial tectonic uplift in the last 0.8 Ma. Long sequences spanning the Jaramillo Subchron (0.99-1.07 Ma) and older Matuyama Chron are recognised at Mangatewaiiti and Mangatewainui in the East Coast region, and Rewa Hill in the Rangitikei Valley. Numerical age control is provided by 40Ar/39Ar single crystal laser fusion ages from plagioclase in key tephra horizons. This new chronology indicates the tephras are nearly twice as old as several previous studies have suggested, thus requiring a major revision of the New Zealand Pleistocene stratigraphy. By integrating isotopic, paleomagnetic and geochemical data, 3 widespread tephras can be correlated between basins of the East Coast and Wanganui: Pakihikura Tephra (ca. 1.6 Ma), Potaka Tephra (1.00 Ma), and Kaukatea Tephra (ca. 1 Ma). These tephras and others provide a chronological framework for much of the early Pleistocene in southern North Island. Potaka Tephra is particularly widespread, allowing correlation between marine strata of the Castlecliffian (local early Pleistocene stage) type section at the Wanganui coast, and marine strata elsewhere in the Wanganui basin, as well as with fluvial and lacustrine strata in the East Coast. The tephra occurs as an ignimbrite and as a catastrophic flood deposit in the East Coast and as a fallout ash in North Canterbury, South Island (ca. 600 km from source). Potaka Tephra (normal polarity) and Kaukatea Tephra (reversed polarity) bracket the top of the Jaramillo Subchron and constrain its age to ca. 1 Ma. This is in accord with the astronomical calibration of the Pleistocene geomagnetic time scale, but older than previous determinations using the 'chronogram' method on K-Ar data. The precise source vents for the distal early Pleistocene tephras are uncertain, however their ages indicates they are coeval with dated proximal ignimbrite sheets from the Mangakino Caldera in the SW part of TVZ. The large number of distal tephras would imply a greater frequency of eruptions from this source than previously expected.</p>

Tephrostratigraphy, Magnetostratigraphy and Geochronology of Some Early and Middle Pleistocene Deposits in New Zealand

<p>Numerous early Pleistocene silicic tephras are exposed in long sedimentary sequences in the East Coast and Wanganui basin regions in southern North Island of New Zealand, some 150-250 km south of the Taupo Volcanic Zone. They provide time planes that can be correlated between different facies and basins. Individual tephras can often be distinguished on the basis of major and trace element glass chemistry, and Fe-Ti oxide composition. Approximately 51 different eruptive events may be recorded in the interval from ca. 1.7 Ma to 0.5 Ma. Early Pleistocene tephras in deep-sea sediments of the Southern Pacific Ocean at latitudes >60 degrees S were previously considered to have been sourced in the TVZ. However, their alkalic compositions are compatible only with volcanoes of Western Antarctica and the Ross Sea region. Most of the tephras examined here are reworked, and many have been emplaced as catastrophic flood deposits in overbank settings of braid plains in the East Coast region. Their mode of emplacement and the presence of ignimbrites in the sequences indicate early Pleistocene transport routes through the site of the present main Axial Ranges, and suggest substantial tectonic uplift in the last 0.8 Ma. Long sequences spanning the Jaramillo Subchron (0.99-1.07 Ma) and older Matuyama Chron are recognised at Mangatewaiiti and Mangatewainui in the East Coast region, and Rewa Hill in the Rangitikei Valley. Numerical age control is provided by 40Ar/39Ar single crystal laser fusion ages from plagioclase in key tephra horizons. This new chronology indicates the tephras are nearly twice as old as several previous studies have suggested, thus requiring a major revision of the New Zealand Pleistocene stratigraphy. By integrating isotopic, paleomagnetic and geochemical data, 3 widespread tephras can be correlated between basins of the East Coast and Wanganui: Pakihikura Tephra (ca. 1.6 Ma), Potaka Tephra (1.00 Ma), and Kaukatea Tephra (ca. 1 Ma). These tephras and others provide a chronological framework for much of the early Pleistocene in southern North Island. Potaka Tephra is particularly widespread, allowing correlation between marine strata of the Castlecliffian (local early Pleistocene stage) type section at the Wanganui coast, and marine strata elsewhere in the Wanganui basin, as well as with fluvial and lacustrine strata in the East Coast. The tephra occurs as an ignimbrite and as a catastrophic flood deposit in the East Coast and as a fallout ash in North Canterbury, South Island (ca. 600 km from source). Potaka Tephra (normal polarity) and Kaukatea Tephra (reversed polarity) bracket the top of the Jaramillo Subchron and constrain its age to ca. 1 Ma. This is in accord with the astronomical calibration of the Pleistocene geomagnetic time scale, but older than previous determinations using the 'chronogram' method on K-Ar data. The precise source vents for the distal early Pleistocene tephras are uncertain, however their ages indicates they are coeval with dated proximal ignimbrite sheets from the Mangakino Caldera in the SW part of TVZ. The large number of distal tephras would imply a greater frequency of eruptions from this source than previously expected.</p>

On multi-sensor monitoring of fiber laser fusion cutting

Laser cutting is a well-established industrial process for sheet metal applications. However, cutting thick plates is still accompanied by problems because of the characteristic limited process parameter window. Since cutting by means of fiber lasers has become dominant, tailored solutions are required in such systems for industrial applications. The development of a robust real-time monitoring system, which adapts the process parameters according to a specific quality requirement, implies a significant step forward towards automated laser cutting and increases the process robustness and performance. In this work, a coaxial multi-sensor monitoring system is tested for fiber laser cutting of stainless steel thick plates. A high-speed camera and a photodiode sensor have been selected for this investigation. Experiments at different cutting speeds, representing primary cut quality cases, have been conducted and various features of the obtained process zone signals have been examined. Finally, the feasibility of industrial application of the developed setup for high-power fiber laser cutting is discussed, followed by several implementation recommendations.